Sure there is a good reason for that, most casinos that are advertised in bitcointalk forum are using Bitcoin as one of the payment methods, and this could be the same with any other business.
I could ask you similar question, is there any rational explanation for many sport clubs to have casino sponsors everywhere, with shirts, stadiums and everything else having much more influence on people of all age.
There is a simple law, companies with most money will continue to advertise, and I see nothing wrong with casinos in bitcointalk.
I am not supporting gambling, but people are not supporting many things and they advertise crap all the time with their clothes, food, etc.